Ensure that dart:js does not attempt to load cached proxies from prototypes.

tests/html/js_test.dart

Index: tests/html/js_test.dart
diff --git a/tests/html/js_test.dart b/tests/html/js_test.dart
index 3d58e2df9fdeafe2ebe467788b42eca45ed4d9ac..39cff4c5cfc43d2315d7d8340be657f2f200bfa8 100644
--- a/tests/html/js_test.dart
+++ b/tests/html/js_test.dart
@@ -186,6 +186,10 @@ function identical(o1, o2) {
   return o1 === o2;
 }
 
+var someProto = { role: "proto" };
+var someObject = Object.create(someProto);
+someObject.role = "object";
+
 """;
   document.body.append(script);
 }
@@ -286,6 +290,17 @@ main() {
       context.deleteProperty('obj');
     });
 
+    group('caching', () {
+      test('JS->Dart', () {
+        // Test that we are not pulling cached proxy from the prototype
+        // when asking for a proxy for the object.
+        final proto = context['someProto'];
+        expect(proto['role'], same('proto'));
+        final obj = context['someObject'];
+        expect(obj['role'], same('object'));
+      });
+    });
+
   });
 
   group('context', () {
@@ -995,4 +1010,5 @@ main() {
 
     });
   });
+
 }